The present invention relates generally to lenses that affect or control characteristics of acoustic waves passing through the lenses and, more particularly, to a lens having an array of independently controllable cells for dynamically controlling propagation of acoustic waves passing through the cells. The lens may be suitable for many applications, including focused ultrasound systems for coagulation necrosis of tissue, such as tumors.
High intensity focused acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors, thereby obviating the need for invasive surgery. For this purpose, piezoelectric transducers driven by electric signals to produce ultrasonic energy have been suggested that may be positioned external to the patient, but in close proximity to the tissue to be ablated. The transducer typically includes a phased array of piezoelectric elements, which are geometrically shaped and positioned such that the ultrasonic energy is focused at a xe2x80x9cfocal zonexe2x80x9d corresponding to a target tissue region within the patient. The transducer elements may be sequentially focused and activated at a number of focal zones in close proximity to one another. This series of sonications is used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a given size and shape.
For such purposes, a spherical cap transducer array, such as that disclosed in U.S. Pat. No. 4,865,042 issued to Umemura et al., which is incorporated herein by reference in its entirety, may be used. A spherical cap transducer array typically includes a plurality of concentric rings disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings generally have equal surface areas and may also be divided circumferentially into a plurality of curved transducer elements or xe2x80x9csectors,xe2x80x9d creating a sector-vortex array. As will be appreciated by those skilled in the art, many other geometric transducer arrays (e.g., flat or linear) may also be employed in a focused ultrasound system. The transducer elements are generally simultaneously driven by radio frequency (RF) electrical signals at a single frequency offset in phase and amplitude. The phase and amplitude of the respective drive signals may be controlled so as to focus the emitted ultrasonic energy at a desired xe2x80x9cfocal distance,xe2x80x9d i.e., the distance from the transducer to the center of the focal zone, and provide a desired energy level in the target tissue region. Notably, such a phased array configuration allows for limited repositioning of the transducer focal zone, without mechanical movement of the transducer itself. Such phased-array transducers also require complex drive circuitry and external amplification hardware that precisely drive each element of the transducer at a certain phase and amplitude in order to generate ultrasonic waves of the proper shape and energy to obtain the appropriate focal zone.
The transducer may be mounted within a fluid-filled casing, such as a table including a chamber that is filled with degassed water or similar acoustically transmitting fluid. The transducer may be connected to a positioning system that moves the transducer within the chamber, and consequently mechanically adjusts the focal zone of the transducer. Alternatively, the positioning system may move the transducer in a horizontal plane perpendicular to the line of propagation, with the focal distance controlled electronically, or other combinations of mechanical and electronic positioning may be used. The top of the table includes a flexible membrane and a fluid-filled bag that may conform easily to the contours of a patient lying on the table. In addition, an imaging device, such as a magnetic resonance imaging (MRI) device, may be provided for monitoring the treatment of a patient.
A patient may be disposed on the table with water, ultrasonic conducting gel, and the like applied between the patient and the bag, thereby acoustically coupling the patient to the transducer. The transducer may be focused towards a target tissue region within a tissue structure, which may, for example, be a cancerous or benign tumor. The transducer may be activated for sufficient time to substantially necrose the target tissue region, e.g., for about ten seconds or more. The transducer may be deactivated, for example, for sufficient time to allow heat absorbed by the patient""s tissue to dissipate, e.g., for about sixty seconds or more. The transducer may then be focused on another target tissue region, for example, adjacent to the target tissue region, and the process repeated until the entire target tissue structure is ablated.
The entire process, i.e., involving a series of sonications necessary to ablate a target tissue structure, may take several hours. As a result, the patient must lie motionless inside the MRI chamber for a long time, subjecting the patient to increased discomfort and possible claustrophobia. As it is, patients generally dislike being in the small MRI chamber, so there is a need to reduce the amount of time needed to ablate tissue, as well as to reduce the treatment time for general concerns such as increased medical costs and surgical risks.
Transducer arrays may be useful for medical imaging (diagnostic ultrasound), non-destructive evaluation (NDE) of materials, and ultrasound therapy devices (high-intensity focused ultrasound). These transducer arrays are composed of numerous transducer elements that are difficult and costly to fabricate and require complex drive circuitry and hardware to power each transducer element. The arrays must have complicated driving hardware in order to provide separate control of the amplitude and phase of the acoustic wave to each element of the transducer. Thus, there is a need for an improved system and method to control the acoustic energy applied to the tissue while doing so in a faster manner.
One approach to simplify control and/or focus of the acoustic energy is through the use of a lens. Extensive work and research have been conducted on so-called xe2x80x9cacoustic lensesxe2x80x9d for decades. Much of this work has focused on solid, mechanical lenses, which usually have been fabricated out of a plastic or wax material. Acoustic waves travel through the lens material at a significantly different speed than in the surrounding medium (usually water). Therefore, the lens material may change the propagational velocity of the acoustic wave as it passes through the lens. Thus, the shape and profile of the lens may be constructed to provide a fixed geometric focus of the incident acoustic waves.
However, in many applications there is a need to control or produce multiple acoustic sources and/or multiple focal ranges and angles of incidence. Although some control of the acoustic beam may be gained by mechanical motion of a fixed focus lens, this approach is typically slow and cumbersome and most applications demand the type of beam control provided by one or two dimensional transducer arrays, which have multiple transducer elements. The presence of multiple transducer elements creates the same problems discussed above of increased complexity, cost and time. Thus, there is a need to provide the benefits and beam control that a phased array transducer provides without its accompanying excessive cost and complexity.
Two previous patents disclose using a voltage dependent material to dynamically alter acoustic transmission through a lens: (1) U.S. Pat. No. 5,546,360, xe2x80x9cUltrasonic Transducer with Lens having Electrorheological Fluid Therein for Dynamically Focusing and Steering Ultrasound Energy,xe2x80x9d Peter Lorraine, 1995; and (2) U.S. Pat. No. 5,477,736, xe2x80x9cElectrically Steered Acoustic Lens,xe2x80x9d Thierry Deegan, 1996. Both of these patents are expressly incorporated herein by reference in their entirety. However, neither of these patents proposes the use or design of a practical phased array lens that may be controlled in real-time to simulate the effect of a phased array transducer.
Thus, there is a need for an acoustic lens that provides the ability to direct and alter the acoustic waves that are transmitted though the lens with the same degree of control provided by a transducer phased array, but offers a significantly simpler and relatively inexpensive design.
The present invention is directed to an electro-dynamic, phased array acoustic lens, and methods of its use. In a preferred embodiment, the lens is provided with an array of cells, each of which is separately controlled to alter characteristics of an incident acoustic wave passing through the lens. By providing for each cell of the lens to independently control the delay and/or amplitude of the incident acoustic wave, the lens may be used in combination with a single-element transducer to control the characteristics of acoustic waves with a similar degree of control, or more so, as that provided by a phased multi-element transducer array. The acoustic lens of the present invention may also be used in combination with a multi-element transducer. In other words, an ultrasonic wave that passes through a given cell of the acoustic lens is phase offset as determined by that cell.
In accordance with one aspect of the invention, an electro-dynamic phased array acoustic lens having an array of cells is provided, wherein each cell may separately control a characteristic, such as, amplitude or delay, of an acoustic wave that passes through the respective cell.
In accordance with another aspect of the invention, an acoustic ablation system is provided with a transducer that outputs an acoustic wave and an electro-dynamic phased array acoustic lens having an array of cells, each cell of which controls a characteristic of the acoustic wave that passes through the cell.
In accordance with a still further aspect of the invention, an acoustic ablation system includes a transducer having a plurality of transducer elements that output acoustic waves, and an electro-dynamic phased array acoustic lens having an array of cells, wherein each cell may separately control a characteristic such as, amplitude, or wave phase (for a continuous periodic wave, or, more broadly, delay, which applies also to pulsed waves) of an acoustic wave that passes through the respective cell. In preferred embodiments, the system employs feedback to determine how to control the characteristics of the acoustic waves and, in particular, to determine where to move the focal zone of the ultrasound energy, and the transducer, if necessary, relative to tissue being ablated.
In accordance with yet another aspect of the invention, there is provided a method of using an acoustic lens to control acoustic waves to ablate tissue, the method including the steps of providing an electro-dynamic phased array acoustic lens having an array of cells and separately controlling one or more characteristics of an acoustic wave that passes through each cell, such as amplitude, phase (for a continuous periodic wave) or wave front propagation delay (for a single pulse), both of these latter characteristics being broadly referred to as xe2x80x9cdelay.xe2x80x9d
In accordance with a still further aspect of the invention, there is provided a method of using an acoustic ablation system to control acoustic waves used to ablate tissue, the method including the steps of providing a transducer that outputs an acoustic wave, providing an electro-dynamic phased array acoustic lens having an array of cells, and separately controlling one or more characteristics of an acoustic wave that passes through each cell. Preferred methods further include employing feedback to determine how to control the characteristics of the acoustic waves and, in particular, to determine where to move the focus of the ultrasound energy, and the transducer, if necessary, relative to tissue being ablated.
Other aspects, advantages and features of the invention will appear hereinafter.